These changes rework the API to be very consistent with the builder pattern. This also introduces better docs and a user's manual mdbook that contains many examples, with more on the way.

Big rocks

• Renderlet renamed to Primitive
• Primitives hold on to their resources so you don't have to
• Stage holds onto resources so you don't have to
  • this has some implications, most importantly that certain resource need to be manually removed from the Stage such as Primitive, AnalyticalLight<T>, Skybox, Ibl
• Skybox and Ibl are now separate types
• AnalyticalLight takes a
• Removed prelude, it was confusing for my editor's LSP
• User's manual!
• New crates/examples crate!